The present invention relates to a process for treating organic substances, particularly waste waters containing chlorinated organic compounds from the production of epichlorohydrin. In this case, the epichlorohydrin obtained by the reaction of dichloropropanol with at least one compound having an alkaline effect, preferably a calcium-hydroxide-containing aqueous solution or suspension, is separated from the reaction mixture by distillation, and a waste water remains as the bottom product which contains saturated and/or unsaturated aliphatic and/or alicyclic chlorinated hydrocarbons, chlorinated ethers, chlorinated alcohols, chlorinated ketones, chlorinated aldehydes and/or chlorinated carboxylic acids. Furthermore, the present invention relates to an apparatus for carrying out this process.
Epichlorohydrin (1-chloro-2,3-epoxy-propane) produced industrially by known processes involving dehydrochlorination of dichloropropanol with agents having an alkaline effect, preferably aqueous solutions or suspensions of calcium hydroxide or sodium hydroxide, at elevated temperatures. The dichloropropanol starting material is preferably obtained as an aqueous solution of the 1,3- and 2,3-dichloropropanol isomer mixture by the reaction of allyl chloride, chlorine and water.
The epichlorohydrin formed according to the above-mentioned process is separated from the reaction mixture by distillation, preferably steam distillation. In this case, an aqueous solution or suspension flows out of the synthesis reactor as the bottom product and, in addition to small quantities of the reaction product, contains further organic compounds, particularly chlorinated organic and inorganic compounds, as by-products of the synthesis as well as unreacted starting material. This bottom product, which occurs as waste water (when calcium hydroxide is used in the epichlorohydrin synthesis as the agent with the alkaline effect), typically contains the following compounds: chlorinated cyclic or acyclic alkanes and alkenes, saturated and/or unsaturated chlorinated aliphatic and/or alicyclic ethers, chlorinated alcohols, chlorinated ketones, chlorinated aldehydes and/or chlorinated carboxylic acids as well as, in addition to other compounds which contribute to the chemical oxygen demand (COD) of the waste water, particularly glycerin and glycerin derivatives, additional calcium chloride, calcium carbonate and possibly excess calcium hydroxide.
The chlorinated organic compounds contained in the bottom product contribute to the overall AOX (adsorbable organic halogen compounds) parameter of the waste water. The AOX is determined as that part of organic halogen compounds (X=F, Cl, Br, I) which can be adsorbed on activated carbon, in which case the entire adsorbed quantity is converted to X=Cl.
Waste waters of this type which contain halogenated organic compounds present a special problem in waste water treatment because, as a result of the high stability of the covalent halocarbon compounds, particularly in the case of sp.sup.2 -bound halogens, removal of these substances requires such high technical expenditures that it is frequently uneconomical.
Known measures for decreasing the content of chlorinated and other halogenated organic substances in waste water include chemical-physical as well as biotechnological processes.
The decomposition of halogenated organic compounds in a biochemical treatment phase in a waste water treatment facility presents various problems: On the one hand, many of these compounds exhibit little or no susceptibility to biological decomposition by means of microorganisms. On the other hand, the concentrations of AOX-generating substances in waste water must not be high and, in addition, should have largely constant values. Furthermore, the volume of activated sludge in facilities of this type is large, and the accumulation of the organic halogen compounds in the sludge presents another problem.
In the prior art, processes for chemical-physical removal of halogen-organic compounds from waste water are therefore preferably suggested, these processes being used as the primary treatment or as a preliminary treatment (with a subsequent biochemical treatment) of the waste water.
Methods which are available include, for example, activated-carbon purification as well as special extraction processes. One disadvantage of these processes is that they produce a secondary product which is contaminated with halogenated organic compounds (loaded activated carbon or extracting agents).
Frequently used measures for eliminating halogenated organic compounds in waste water include chemical-thermal processes. These include the so-called wet-oxidative processes in which halogenated organic compounds are decomposed in an oxidizing atmosphere at high temperatures and substantially elevated pressures. Although this method is very effective, it is also very cost-intensive because of the high energy consumption and the expensive equipment required.
To reduce the extreme physical conditions of chemical-thermal processes, the prior art suggests the use of catalytically active compounds, in which case such substances may be introduced into the system which is to be dehalogenated either through addition of corresponding reagents or they may form as intermediate products during the decomposition reaction.
Certain metals, metallic hydrides or metallic alcoholates, individually or in combination with a strong inorganic base, for example, are used as substances which have a high reactivity with respect to organically bound halogens.
One disadvantage of the known chemical-physical processes for the decomposing or destroying halogenated organic compounds relates to their relatively high cost which results particularly from the consumption of expensive reagents and from the provision of an oxidizing or inert atmosphere as well as relatively high temperatures and pressures and from the related requirement of expensive equipment. In addition, when the economic efficiency of the known methods for the dehalogenation and/or dehydrohalogenation of halogen-organic compounds is calculated, the often long reaction times (often more than ten hours) and the frequently only moderate decomposition rates have an unfavorable effect.
Furthermore, processes for treating waste waters from pulp bleaching are known in which the lignin chloride compounds contained in the waste water are partially dehalogenated and/or dehydrohalogenated while maintaining certain temperatures, pH-values and residence times. Correspondingly, an expensive three-step process is known from published German Patent Application No. DE 3,620,980 which envisions a preliminary treatment of the waste water by precipitation methods and a thermal hydrolysis step involving a pH setting of 11.5 with lime water and/or NaOH, a temperature setting of from 40.degree. to 70.degree. C., and a residence time of from 1 to 3 hours. According to PCT Patent Application No. WO 92/05118, the decomposition of lignin chloride compounds is carried out at pH-values of from 6 to 11, at temperatures of from 90.degree. to 150.degree. C., an overpressure of from 70 to 475 kPa during a residence time of from 2 to 5 minutes, in which case, however, AOX decomposition rates of more than 60% may be obtained, preferably with the use of additional chemical reagents or by a targeted mixing of different waste waters from pulp bleaching (in which case catalytic processes probably play a role).
The processes suggested here contain no reference to their possible use in treating waste waters from the synthesis of epichlorohydrin. In addition, it is impossible to use the aforementioned methods for decomposing lignin chloride compounds in waste water from pulp bleaching in a treatment process for waste waters from the synthesis of epichlorohydrin, since AOX decomposition rates of over 50% cannot be achieved in this manner because of the completely different composition of the waste waters and the therefore non-transferable parameters with respect to the pH-value, the temperature, the pressure and the residence time.
Thus, there exists an urgent requirement for at least partially removing the chlorinated organic compounds which occur in the waste water of a facility for the synthesis of epichlorohydrin in order to meet environmental goals. This requirement is documented in various patent applications in this technical field, for example, in published European Patent Application No. EP 247,670; in published German Patent Application No. DE 3,016, 667; and in published German Patent Application No. DE 3,520,019. However, the processes suggested in these prior patents either comprise high-cost purification stages for separating the undesired chlorinated organic compounds, or they modify the process for producing epichlorohydrin to such an extent that the amount of formed chlorinated organic by-products can be minimized. In each case, the proposed measures which are cost-intensive and often result in an only unsatisfactory reduction of the organic chlorine compounds in the waste water.
Thus, taking into consideration the aforementioned prior art, there remained a need for an economical process for removing halogenated organic compounds from industrial waste water which could be implemented with low technical expenditures, was as simple as possible to carry out, consumed low amounts of reagent chemicals, and produced high AOX decomposition rates at lower temperatures and pressures as well as short residence times. In particular, there remained a need for a process for treating waste water from the synthesis of epichlorohydrin.